Concrete examples of OpenCog's functionality
It depends what you mean by "what OpenCog can do?". 

OpenCog, at a high-level, is a loosely coupled collection of various theoretical and variants of conventional methods aimed at constructing the beginnings of an AGI.

With that said, it's purely applied uses are fairly limited. It can do some typical NLP and ML tasks if used correctly, albeit, it will almost always be ineffective when compared to a problem specific solution. However, this is understandable, as OpenCog is not geared at solving narrow problems.

OpenCog, as well as Goertzel's other projects(SingularityNet, AGI society), are broad, top-down attempts at formulating an AGI system.
Outside of AGI circles they are also not oft referenced or researched(although, I am utilizing some aspects of the system in work I am currently crafting).